The geochemical behavior of Cu and its isotopes in the Yangtze River

Copper (Cu) isotopes can be a useful tool to constrain the interaction of water and the environment, but they have not been widely applied to riverine research in the preceding decades. Isotopically heavy Cu in rivers (global average: about +0.7‰) compared to rocks (at about 0‰) has been attributed to: a) the mobilization of heavy Cu during oxidative weathering, and b) partitioning between an isotopically heavy, organically complexed dissolved pool, and an isotopically light pool adsorbed to particulates. Here, we report Cu concentrations and isotope ratios of the main stream of the Yangtze River and its several tributaries. We find that the Yangtze River exhibits anomalously heavy Cu isotope compositions compared to other rivers: δ⁶⁵Cu_{NIST 976} of dissolved Cu for the main stream, from Chongqing to Nanjing, ranges from +0.59 to +1.65‰, while the tributaries vary from +0.48 to +1.20‰. A negative correlation is observed between Cu concentrations and Cu isotope compositions. We attribute the anomalous Cu isotope geochemistry of the Yangtze River to two key features of the basin: first, the influence of the Three Gorges Dam (TGD), and second, the presence of extensive Cu sulphide deposits close to the lower reaches of the river. In the upper reaches, downstream towards the TGD, δ⁶⁵Cu values increase as Cu concentrations decrease, reflecting the preferential adsorption of light Cu by sedimenting particulate phases. δ⁶⁵Cu values continue to increase to a maximum of +1.65‰ in the middle reaches, at Guangxingzhou. The lower reaches, from Jiujiang to Tongling, are characterized by less positive values of δ⁶⁵Cu (at about +0.60‰), due to the oxidative weathering of Cu sulphide deposits. The overall Cu-δ⁶⁵Cu trend in the river reflects mixing of these waters from the lower reaches, influenced by Cu sulphides, with waters from upstream, which have lower Cu concentrations and elevated δ⁶⁵Cu values.